MELBOURNE, Australia – The El Niño Southern Oscillation (ENSO) is currently neutral. However, models suggest the tropical Pacific Ocean will continue to cool, making the chance of a La Niña forming in late 2017 at least 50%; around double the normal likelihood.
While this means the Australian Bureau of Meteorology’s ENSO Outlook has shifted to La Niña WATCH, rainfall outlooks remain neutral due to competing climate drivers.
Following a brief period of warming, tropical Pacific surface waters cooled significantly in the past fortnight, and hence the central to eastern tropical Pacific Ocean is now generally cooler-than-average.
Atmospheric indicators of ENSO, including the Southern Oscillation Index (SOI), trade winds and cloudiness near the Date Line, are also approaching La Niña levels.
Seven of the eight international climate models surveyed by the Bureau suggest that sea surface temperatures will reach or exceed La Niña thresholds by November 2017.
However, indicators need to remain at La Niña levels for at least three months to be considered an event.
This is forecast by six of the eight models. If a La Niña does occur this year it is likely to be short and weak, as sea surface temperatures are forecast to warm again in early 2018, as the austral autumn is the time when La Niña events normally decay.
The Indian Ocean Dipole (IOD) is currently neutral. All six international climate models indicate neutral conditions will continue through 2017. Indian Ocean Dipole events are typically unable to form between December and April due to the influence of the monsoon trough over the tropical Indian Ocean.
La Niña events typically bring above average rainfall to eastern Australia during late spring and summer.
However, given the competing influence of other climate drivers (weakly warm waters to the north of Australia, and cooler waters in the eastern Indian Ocean), current climate outlooks do not favour widespread rainfall across Australia for November to January.
Weak La Niña events in summer can also produce heatwaves in southeast Australia.
